Jack backend does not handle sample formats correctly

I am currently trying to port a synthesizer program from rtaudio to libsoundio.
The synth is written largely in fixed-point integer and outputs signed 16 bit samples.

Now when I request the sample format S16NE and I also use the Jack backend, the audio output is garbage. I know Jack jack accepts only float data, but I act as if I do not know being user of the library. (previously rtaudio did automatic sample conversion from the program to Jack)

I request the format like suggested on the main page example, you can see the steps in the source linked for reference.
https://gist.github.com/jpcima/96f842a1d78d643e70d4531ac2ef3acc

I tested the following scenarii:
1. using Pulseaudio, either int16 or float sample type -> OK
2. using Jack, float sample type, or format not specified -> OK
3. using Jack, int16 sample type -> NOT OK

I would expect from the library either of these outcomes, but none happen.
1. emitting an error
2. automatic sample conversion (rtaudio behavior)
3. change stream->format whatever is supported, and make the user is responsible for converting manually.